We propose to continue to develop optical methods for monitoring membrane potential. In particular, we plan to work in two directions. The first is to develop more sensitive dyes to make the method easier to use. The second is to develop the apparatus for monitoring activity in CNS neurons. At present we can monitor activity (action potentials) in about 50 neurons simultaneously. We hope to improve the apparatus so that 200 neurons can be monitored. This would be used to study the neuronal basis of food aversion learning in the mollusc Aeolidia papillosa.